This invention relates to a semiconductor device and, more particularly, to a bipolar transistor having a thin base region and a method of manufacturing the same.
A bipolar transistor is known to operate at a higher speed with decreasing thickness of the base. Besides the thin base, the smaller the transistor size is, the smaller the parasitic capacitance and resistance becomes, resulting in more higher operation speed.
An ion implantation method has been ordinally used to form the thin base. However, the ion implantation method has limitations on injecting impurity ions very shallowly. Moreover, it is unavoidable that the ion implantation method needs an annealing process after ion-implanting to retrieve the littice from defect. The annealing process diffuses the injected impurities to thicken the base region.
Therefore, it has been proposed by K. Suzuki et al in Symp. on VLSI Technol. pp. 91-92 (1989) titled "50 nm Ultra-Thin Base Silicon Bipolar Device Fabrication Based on Photo Epitaxial Growth" to utilize the low-temperature epitaxial growth method to form a thin base layer. While the proposed process provides a thin base layer, an opening for forming an emitter region into the base layer is selectively formed by photolithography process into an insulating layer covering the base layer. It is well known that the photolithography process has mask-misalignment problem unavoidably. The plane size of base layer thereby requires to be large. As a result, parasitic capacitance and resistance become large to ristict the improvement in switching speed.